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W3C XML Schema, commonly called XSD, is a language for describing and constraining classes of XML documents. An XSD schema sets out rules for a document’s elements, attributes, content, datatypes, and values, so an XML application can assess whether a document conforms to those rules. It can also document what document components mean and how they are used.
What is an XSD schema used for?
An XSD schema provides a formal description of the XML documents an application expects. It can define which elements and attributes are allowed, how content is organized, and which datatypes and values are valid. Schemas may also specify normalization and default values.
Because an XSD schema is itself written using XML vocabulary and namespaces, it can express rules about XML while remaining an XML document. The W3C describes its purpose as defining and describing a class of XML documents by constraining and documenting the meaning, use, and relationships of their components. See the W3C XSD 1.1 Structures specification.
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A processor can assess an XML document against a schema’s structural and datatype constraints. A schema may, for example, specify that an element is expected, constrain the content it can contain, or declare an attribute’s datatype and permitted values.
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Datatype rules distinguish a value’s value space—the values themselves—from its lexical space—the textual forms used to write those values. XSD defines mappings between these spaces and rules relevant to checking datatype validity. These concepts are specified in W3C XSD 1.1 Part 2: Datatypes.
Schema validity is not a guarantee that a document meets every business or application requirement. The W3C notes that some applications need checks beyond what XSD can express. Applications may therefore need additional validation for rules that depend on context or other requirements not captured by the schema.
What are the parts of XSD 1.1?
XSD 1.1 is organized into two specifications, each addressing a different part of schema processing.
| Part | What it defines |
|---|---|
| Part 1: Structures | Schema components, how schemas are represented and composed, and how documents are assessed for schema validity. |
| Part 2: Datatypes | Datatype concepts and the rules used to check datatype validity. |
The W3C lists both XSD 1.1 parts as Recommendations dated 5 April 2012; see its XML Schema publications list. The W3C describes version 1.1 as intended to remain mostly compatible with version 1.0 while correcting errors and making improvements. Its overview records the original XSD 1.0 Recommendation date as 2 May 2001 and the second edition as 28 October 2004; see the W3C XML Schema overview.
Is XML Schema the same as XSD?
In common technical usage, “XML Schema” refers to the W3C schema language, while “XSD” is the usual abbreviation for XML Schema Definition. An XSD schema is a schema written in that language. The word “schema” can also refer more generally to a set of rules for a document, so context matters when comparing XSD with other XML validation approaches.
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The W3C Structures specification is primarily a precise language-definition reference, not a tutorial. For a step-by-step introduction, the W3C recommends its non-normative XML Schema Primer. When choosing an XSD approach or processor, check that the processor supports the XSD version and features your schema requires; the specifications define the language, but do not establish support for every processor.
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